Interpretive Summary: Phytoplasmas are tiny bacteria that cause many serious diseases in plants around the world. Each phytoplasma cell is surrounded by a single membrane. In this way, they are like mycoplasmas-their relatives that cause diseases of animals, and spiroplasmas-relatives that include pathogens of plants. Phytoplasma cells are among the smallest cells known. In addition, phytoplasmas have some of the smallest known genomes, that is, they possess a very small number of genes. To understand how phytoplasmas survive inside their plant and insect hosts and also cause diseases in spite of their small gene complement, we are comparing genes in phytoplasma with genes in other bacteria, including mycoplasma and spiroplasma. We have found that genes needed for the synthesis of an essential vitamin were present in ancestors of phytoplasma but have been destroyed in phytoplasma. We have also found that phytoplasma contains a gene for a protein-digesting enzyme that may aid the phytoplasma to invade its hosts. These findings will be of interest to scientists who wish to understand the evolution of phytoplasmas and the mechanisms by which phytopalsmas cause disease.

Technical Abstract:
Technical abstract:
Phytoplasmas are wall-less phytopathogenic prokaryotes of small genome size that are obligate parasites of insect vectors and plant hosts. We have cloned a clover phyllody (CPh) phytoplasma DNA locus containing five potential coding sequences. Two were identified as pseudogenes (YfolP and YfolK) homologous to folP and folK genes, which encode dihydropteroate synthase (DHPS) and 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK), respectively, in other bacteria. Evolution of the phytoplasma presumably involved loss of functions through the formation of these and other pseudogenes during adaptation to obligate parasitism. The findings suggest that the phytoplasma lacks capacity for de novo folate biosynthesis and possesses a transport system for absorption of preformed folate from host cells. The YfolP-YfolK region was flanked by three open reading frames (ORFs) encoding a DegV family protein, a hypothetical protein with a P60-like lipoprotein domain homologous with the P60-like Mycoplasma hominis protein, and a glycoprotease (Gcp) protein that possibly functions as a host adaptation or virulence factor.